A systematic investigation of the factors influencing the reduction of iron (III) and iron (II) porphyrins is being undertaken. Changes in the redox mechanism as a function of porphyrins have been prepared with eight different anions ranging from the non-coordinating C104 negative anion to the tightly held F negative anion. A 700 mV stabilization of the positively charged iron center has already been observed as the coordinating ability of the counterion increased. The degree of axial complexation to the iron center by sterically hindered and unhindered nitrogenous bases is being investigated as a function of the associated ion pair. The extent of ion pair coordination is found to directly influence not only the strength of the adduct formed, but also the mechanism of electroreduction. Rates of heterogeneous electron transfer will be determined for each of the electron transfer processes, including the binuclear oxo- and nitrido bridged porphyrins. Half wave potential measurements will be made over a wide range of temperatures. From the shift of E1/2 with T the entropy of electron transfer may be calculated. Comparisons will be made between the electron transfer entropy and the entropy of any spin change or ligand addition. A mechanism involving spin equilibrium and electroreduction will be proposed.